Many industrial processes now in operation require the generation of temperature in the region of, or in excess of, 1000.degree. K. Traditionally, such temperatures have been attained by means of furnaces operating on the principle of chemical combustion wherein the process is conducted in the presence of a combustion flame. Such furnaces have the disadvantage of involving the introduction of combustion materials and combustion products into the process.
Of recent years furnaces using a heat source comprising an electrical discharge have come more into consideration. Such furnaces may be, for example, arc furnaces or may be, for example, "plasma" furnaces in which discharge at an electrode heats a flow of inert gas into a heating chamber. Furnaces of either type can provide temperatures in excess of 5000.degree. K. although the area in which they are mainly under development is in the temperature range of about 1500.degree. K. to 3000.degree. K. since at such temperatures the physical problems of providing a structure for the containment of the electrical discharge are more easily solved than at higher temperatures. In the field of ore, or ore derivative, processing the last mentioned range is of particular interest since it is below the temperature at which iron starts to volatalise.
The electrical insulation of electrical discharge furnaces, or of parts thereof from the remainder of a furnace, has proved to present a problem which, unless solved, greatly reduces their efficiency. This problem arises from the fact that many materials normally used, or of potential use, in furnace construction as electrical insulators can become electrically conductive to varying degrees at the temperatures involved in electrical discharge furnace operation, for example, at temperatures in excess of 1500.degree. K.
Because of the difficulty in insulating the discharge source, for example an electrode, from the surrounding furnace structure, it is known to space the electrode from the furnace walls. There is a tendency for unwanted sporadic electrical discharge over the resulting gap and this may be a source of wear of the electrode structure resulting in reduced electrode life. Such wear may be particularly serious in plasma furnaces where the electrode assembly may be a complicated and expensive part of the furnace.
The visual and infra-red radiation inside an electrical discharge furnace is intense and efficient thermal insulation of the heating zone is necessary not only for efficiency but to enable the economic construction of at least some parts of the furnace not directly exposed to such radiation from materials not capable of withstanding the full effects thereof.
One possibility for reducing the problem of electrode wear due to sporadic discharges across the spacing between it and the furnace walls is to increase that spacing. However, this may allow the direct escape of radiation from the heating zone and the exposure to such radiation of the mechanical structure supporting the electrode and is therefore not always a practical solution to the problem. This may be particularly so where a moveable electrode is employed and the supporting structure incorporates mechanical linkages which may be prone to heat distortion.
The problems outlined above cannot be cured satisfactorily in practice solely by direct cooling to counteract the loss in insulating properties. This is because only the bulk of material of construction of the furnace immediately adjacent to the cooling means becomes non-conductive and, therefore, the cooling means has to be positioned in the material of construction very close to the heat-exposed surface. This results in potential structural weakness in the furnace and a high rate of power loss by heat transfer. Heat transfer rates are also, generally, not high enough to reduce the temperature of the material sufficiently to obtain the desired result.